A photolithography system has as its basic components an illuminator with a light source, a patterned reticle, a projection imaging lens and a photosensitive (e.g., photoresist-coated) wafer. The illuminator illuminates the reticle with light from the light source. Light transmitted by or reflected from the reticle is then imaged by the projection imaging lens onto the photosensitive wafer. The photosensitive wafer is then processed to form a pattern on the wafer. The photolithographic exposure process and post-exposure process are repeated with a number of different reticles to form on the wafer a semiconductor structure that defines an integrated circuit.
Illumination systems in lithography tools generally employ Hg lamps as the light source. However, the emission from an Hg lamp is very non-uniform. To obtain the exposure uniformity required for photolithography, the emission from the Hg lamp can be directed through a homogenizer rod. This involves either placing the lamp very close to the homogenizer rod or collecting the emission with lenses and imaging the emission onto the input surface of the homogenizer rod.
The light entering the homogenizer rod bounces between the longitudinal surfaces by total internal reflection, and eventually exits the opposite end. The homogenizer rod length is selected so that that the output is very uniform. The length of the rod is determined by a number of factors, such as the homogenizer cross-sectional area, the angular spread in the light rays traveling through the rod, the number of internal bounces needed, and the uniformity required for the particular application.
Typically, the light must bounce between opposite walls of the rod a minimum of 5 times. The larger the number of bounces between the input and the output, the better the illumination uniformity at the output. The more typical implementation is to use a collecting lens to collect the light from the source and image it onto the input surface of the homogenizer rod.
Unfortunately, Hg arc lamps have short operational lives, typically measured in weeks or months. In addition, they are inefficient, with only a few percent of the input power actually emitted within the desired optical spectrum. Moreover, the disposal of Hg arc lamps is an environmental concern because the Hg must be disposed of carefully and in accordance with regulatory requirements.
Hg lamps are also limited in output power. To increase the throughput of lithography tools, it becomes essential to increase the power emitted from the output face of the homogenizer rod. Because the source size and angular emission are defined by the physical characteristics of the homogenizer rod and by the angular emission (determined by the lens coupling the source to the rod), the source etendue is determined. Increasing the power emitted from the output of the homogenizer rod is equivalent to increasing the source brightness.
Increasing the power of an Hg light source usually comes with the price of increasing the source size. Doubling the output power generally requires doubling the source size. As a result, the effective brightness of the source remains approximately constant and the power density at the wafer plane remains constant.
Consequently, the throughput is generally not improved with these larger lamps because the larger power cannot be relayed to the wafer plane. Decreasing the system etendue while maintaining the amount of emitted power from the mercury lamps has been equally difficult to achieve.